Intermittent Fasting: 8 Facts You Should Know

Intermittent fasting (IF), or eating only during specified periods, is a weight-loss method that has been attracting attention since 2012, when it was first introduced in Dr. Michael Mosley’s TV documentary Eat Fast, Live Longer. In the years since IF was first proposed, it has continued to draw attention in both popular science and academic research circles. However, as with any specialized diet, it isn’t for everyone, and those who are considering it should carefully evaluate whether it’s a healthy option for their particular weight loss journey. 

1. Intermittent fasting can help with weight loss.

Multiple studies over the past decade have demonstrated that intermittent fasting (IF) is an effective method of losing weight. For those who have found calorie-restricted diets challenging to maintain, IF may be a more effective option. New research now indicates that the timing of the fast—particularly cutting out late-night eating—is a particularly important factor for success on the eating plan.

That said, it is unclear that intermittent fasting is equally or more effective than traditional diets centered on strategic calorie cutting. A 2017 study published in the Journal of the American Medical Association compared the weight loss of two groups of obese individuals—one practicing calorie restriction and one alternate-day fasting—as well as a control group. The weight loss of the two groups over the course of one year was nearly identical (6% and 5%, respectively). Additionally, the dropout rate was higher among the fasting group.

Also problematic for intermittent fasters is the tendency to binge during eating periods. After a long period of fasting, during which one can ingest only calorie-free liquids, it can be easy to overdo it once the fasting window has closed.    

2. It can take the body two to four weeks to become accustomed to intermittent fasting.

Although intermittent fasting can be an effective weight-loss method, you shouldn’t expect to see results immediately. You’re likely to feel hungry—and “hangry”—as you get accustomed to your new routine. Those who continue the diet past the tough initial weeks are likely to keep going because they then begin to feel better and see results. Because it takes a bit of getting used to, you may want to wait to try it until you’re having a period of down time (rather than at the beginning of the school semester or when a big report is due at work, for example).

3. There are several methods of intermittent fasting.

Many variations on the IF diet have been promoted, each with their share of devotees. Some, such as the 16/8 method, require eating during a certain number of hours and abstaining for the rest of the day (in this case, 8 and 16 hours, respectively). The 5:2 method, in contrast, focuses on restricting caloric intake to 500 (about a quarter of the average daily amount) during two days of the week.

Still other variations, such as Eat Stop Eat and alternate-day fasting, involve fasting for one or more nonconsecutive days of the week. Such diets center on the principle that fasting for 24 hours or more pushes the body into “metabolic switching,” when it begins to burn fat rather than sugar as an energy source.

Last, fans of Paleo eating may be drawn to the Warrior Diet, which encourages you to eat like an ancient warrior: sparingly for 20 hours, then as much as desired for the remaining four hours. If you are interested in giving intermittent fasting a try, speak with your doctor about the pros and cons of the different methods to decide which aligns best with your health needs.

4. Intermittent fasting can improve blood sugar control.

A recent pilot study published in the World Journal of Diabetes found that restricting food intake to a four- to eight-hour window each day for a period of two weeks improved fasting glucose levels for ten individuals with Type II diabetes. Prediabetics may benefit significantly as well, as demonstrated in a 2018 study published in Cell Metabolism.

Here, researchers monitored a group of men following a diet of Early Time-Restricted Eating (eTRF) for five weeks to determine whether IF could have health benefits independent of weight loss, in particular with regard to blood sugar levels. They found that not eating after 3 p.m. each day increased insulin sensitivity while lowering blood pressure and oxidative stress.    

5. It’s not for everyone.

While intermittent fasting has proven to be a positive dieting alternative for some populations, there are certain groups for whom it is not a healthy option. Individuals who have a history of or are at risk of having an eating disorder should never try intermittent fasting. This practice can be dangerously linked with bulimia, for example, because of the common impulse to binge after fasting. Women who are pregnant or breastfeeding should also avoid dietary practices involving fasting unless under the close supervision of a doctor, as they need to maintain a consistent caloric intake for their and their baby’s health.

As mentioned in no. 4, prediabetics and some Type II diabetics may experience positive health outcomes from intermittent fasting. However, for individuals with advanced diabetes and those who are taking medication for diabetes, it is also risky to attempt intermittent fasting without being monitored by a doctor.

Last, no one under the age of 18 should engage in intermittent fasting, as it could interfere with physical development and disrupt healthy eating habits during an impressionable stage.

6. It’s still important to eat sensibly when you are in the eating part of the IF cycle.

Although the flexibility of not having a prescribed diet is certainly part of the appeal of intermittent fasting, the eating portion of the cycle shouldn’t be seen as a free-for-all. While you can certainly treat yourself to a decadent treat or two, focusing your diet mostly on whole foods, fruits and vegetables, lean proteins, and healthy fats is still recommended. The Mediterranean Diet is frequently mentioned as a positive example of how to eat during IF.

7. Research shows that intermittent fasting may have additional health benefits outside of weight loss.

Mark Mattson, Ph.D., a neuroscientist at Johns Hopkins who has studied intermittent fasting for 25 years, notes that in prehistoric times, humans evolved to go for long periods without eating when hunting and gathering, and that we still have the ability to do so. According to his research, intermittent fasting may have a host of benefits, including improved memory and heart health, maintenance of muscle mass during fat loss, and a reduction in tissue damage after surgery (in animals).  

8. It’s important to practice good self-care while intermittent fasting.

Intermittent fasting isn’t easy. If you’ve decided to try it, be kind to yourself as you adapt to a method of eating that may feel foreign and out of sync with the world around you. Particularly on your fasting days, remember to drink enough liquids to stay hydrated (water, black coffee and tea are allowed). You may find that your energy for exercise is limited, so tread carefully when you begin an exercise session and get plenty of sleep. Above all, if you have a health condition or any questions about how to begin a healthy intermittent fasting regimen, speak with your doctor before getting started.